Studies of the purification and mechanism of action of a plasma protein, the Cl inhibitor, have continued. Last year we reported that we had developed a new method for preparing this difficult to purify plasma protein. We have now been studying the mechanism of action of this protein. Previous studies have suggested that this protein interacts with activated Cl to form a covalently bound complex Clr-Cls (C1 1NH) . How this complex forms is unknown. Formation of the complex is associated with disassociation of Cl and release of Cl from its target binding site. We have now shown that this result is target specific. The published result is correct when one studies antibody coated sheep red cells, the usual target examined. If one studies other targets including guinea pig red blood cells and some viruses like para influenza virus one finds that Cl inhibitor remains target bound. Analysis of this effect suggests that a complex has been formed of Cl INH and Cls. Presumably this represents the first step in the formation of the large complex formed with sheep cells. This type of analysis will allow us to understand the various steps involved in Cl 1NH mechanism of action. Studies continue on the mechanism by which bactericidal antibody influences complement mediated lysis of targets. It appears that complement directs C4 binding to a particular viral surface protein, the HN protein, that does not bind C4 in the absence of specific anti HN antibody.